Optical transmission systems including optical amplifiers, apparatuses and methods

ABSTRACT

Optical systems of the present invention include amplifiers configured to achieve maximum signal channel in a span downstream of the transmitter and amplifier site and to decrease the interaction between the wavelengths at high signal channel powers. In addition, the system can include various types of optical fiber positioned in the network to provide for increased signal channel powers and higher gain efficiencies in the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part (“CIP”) of commonlyassigned U.S. Provisional Application No. 60/127,665 filed Apr. 2, 1999,which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] The present invention is directed generally to opticaltransmission systems. More particularly, the invention relates toamplifying optical signals in optical transmission systems andcontrolling nonlinear interactions between signal channels, as well asthe amplifier spacing in the optical systems.

[0004] Optical communication systems transmit information by generatingand sending optical signals corresponding to the information throughoptical transmission media. Information transported by the opticalsystems can include audio, video, data, or any other information format.The optical systems can be used in long distance and local telephone,cable television, LAN, WAN, and MAN systems, as well as othercommunication systems.

[0005] Information can be transmitted optically using a broad range offrequencies/wavelengths at high data rates and relatively low cost,which are desirable attributes for high capacity, transmission systems.Also, multiple optical wavelengths that are combined using wavelengthdivision multiplexing (“WDM”) techniques into one optical signal thatcan be transmitted through one optical fiber, which further increasesthe data carrying capacity of optical systems. As such, optical fibertransmission systems have emerged as a cost-effective alternative toelectrical systems for providing high capacity, communication systems.

[0006] However, optical transmission systems are not free fromattenuation and various forms of degradation that limit the performanceof the systems. For example, optical fiber is not a perfect transmitterof electromagnetic radiation in the optical spectrum. Thus, theintensity of an optical signal will attenuate as it travels through thefiber, due to scattering from fiber material imperfections and otherdegradation mechanisms. In addition, optical noise, from signalattenuation, chromatic dispersion, nonlinear interactions, and othersources, will accumulate and propagate in the fiber along with thesignal further degrading the quality of the signal. Furthermore, opticalsystems generally are not operated in the identical manner, whichrequires that interfaces be provided to interconnect different opticalsystems.

[0007] It is therefore necessary to regenerate optical signals beingtransmitted through the optical system to overcome the three primarylimitations on optical transport, namely: 1) optical signal attenuation,2) optical noise accumulation, and 3) optical system interconnectivity.The regeneration of optical signals can be performed either optically orelectrically.

[0008] The development of optical amplifiers greatly reduced the cost ofoptical systems, and WDM systems in particular, by essentiallyeliminating the need to electrically regenerate signal merely toovercome signal attenuation. While the development of optical amplifiershas greatly reduced the equipment costs associated with amplifiers inoptical systems, there remain substantial operational costs. Real estateand building acquisition and maintenance costs associated with opticalamplifiers can be a sizable portion of the optical system operationalcosts, which suggests maximizing the distance between optical amplifiersin an optical system. However, maximizing the distance betweenamplifiers can reduce the maximum distance that optical signals can betransmitted before having to be regenerated to overcome accumulatedoptical noise.

[0009] Additional cost savings in the system can be achieved byincreasing the transmission capacity of existing optical fiber comparedto installing additional fiber in the system. Therefore, it is desirableto increase the information bit transmission rate in the fiber and thenumber of wavelengths used to carry information to increase theinformation carrying capacity of the fiber. However, increasing the bitrate of each channel generally requires the channel spacing to beincreased. Furthermore, increasing the bit rate and/or the number ofchannels often requires that the transmission distance to be decreased,which increases the system cost.

[0010] The ability to increase the capacity of the system is limited bythe optical signal degradations that occur in the system. Optical signaldegradation occurs via numerous mechanisms during transmission inoptical fiber. The primary mechanisms are chromatic dispersion andvarious nonlinear interactions, such as four wave mixing. Thedegradation caused by these mechanisms increases proportionally aseither the bit transmission rate or the number of the channels within awavelength range is increased.

[0011] Generally, information carrying signal wavelengths, or signalchannels, are launched into the optical fiber at a maximum signalchannel power. The signal channel power decreases as it travels throughthe fiber until it reaches a minimum signal power, at which time it mustbe amplified to prevent degradation of the signal. Thus, for a givenchannel spacing, the maximum signal launch power, minimum signal power,and the attenuation of the fiber establish the maximum amplifier spacingin the system.

[0012] The maximum signal launch power is limited to powers below whichnonlinear interactions do not cause unacceptable signal degradation. Thespacing of the channels as well as other factors, such as the signalchannel polarization, affect the maximum signal launch power. Safetyconcerns may further limit the total power that can be launched into thefiber. The minimum signal power is determined based on the minimumacceptable signal to noise ratio required to reliably transmitinformation through the system.

[0013] The development of optical fiber technology has resulted in fiberhaving very low attenuation levels (0.25 dB/km) compared to older fiberdesigns (>0.30 dB/km) in the wavelength range around 1550 nm. The lowerloss fiber allows amplifiers to be separated by greater distances forsignal transmitted at a given power level and/or lower power signals tobe transmitted over greater amplifier spacings.

[0014] The different optical fibers used in systems introduce differentamounts of chromatic dispersion as a function of optical wavelength.Chromatic dispersion in standard single mode, silica-based, opticaltransmission fiber, such as SMF-28, generally varies as a function ofwavelength. Average dispersion values in standard silica-based fiber areapproximately −17 ps/km/nm in the 1550 nm low loss optical transmissionwindow, whereas the wavelength at which zero dispersion occurs (the“zero dispersion wavelength”) is typically around 1300 nm.

[0015] In optical transmission systems employing standard transmissionfiber, chromatic dispersion can severely degrade the optical signalquality and thereby limit the maximum transmission distance. Numerousmethods have been developed to effectively counteract dispersion in thestandard fiber. For example, dispersion compensating (“DC”) fibers havebeen developed that have high dispersion rates, on the order of 10²pm/km/nm, that are opposite in sign from transmission fiber. The DCfibers can be inserted into the transmission fiber at various locationsto maintain the absolute dispersion in the system to within a desiredrange.

[0016] New transmission fibers have been designed to minimize thechromatic dispersion in the 1550 nm window. The new fiber types,generally referred to as low dispersion (“LD”) fiber, have much lowerdispersion than standard fiber in the range ±5 pm/km/nm for non-zerodispersion shifted (“NZ-DS”) fiber, such as Truewave, LEAF, and LS, andeven lower for zero dispersion shifted (“DS”) fibers. The LD fibersfacilitate the optical signal transmission over substantially longerdistances before substantial signal degradation occurs as a result ofchromatic dispersion. In addition, DC fibers also have been developed tocompensate for dispersion in LD fibers.

[0017] However, a problem with LD fiber arises from the interrelation ofchromatic dispersion and nonlinear interactions. High rates ofdispersion tend to decrease nonlinear interactions between closelyspaced wavelengths, because the relative velocity of the channels, or“walk-off”, decreases the interaction time between the channels. In LDfiber there is much less walk-off between adjacent closely spacedchannels resulting in longer interaction time between channels, whichincreases the nonlinear interaction and resulting degradation of thesignal channels.

[0018] Nonlinear interactions in LD fiber can be decreased in the systemby increasing the spacing between adjacent channels or decreasing thesignal channel power. However, increasing the channel spacing generallyreduces the total number of channels in the system. Likewise, decreasingthe signal channel power generally requires a corresponding decrease inthe amplifier spacing and the transmission distance between electricalregeneration sites.

[0019] The limitations imposed by nonlinear interactions becomeincreasing problematic for LD fibers at relatively low transmissioncapacities and short distances. The current methods of reducing thenonlinear interactions in LD fiber greatly decreases the optical systemcapacity and performance compared to equivalent systems employingstandard fiber.

[0020] The capacity limitation resulting from non-linear interactions isnot limited to LD fiber, but also plagues standard fiber, transmissionsystems. This limitation will require increased capital and operatingexpenditures to expand optical communication system capacity.Accordingly, there is an extremely pressing need to overcome the fiberlimitations in current optical systems to allow continued growth incommunication systems and communications based technology.

BRIEF SUMMARY OF THE INVENTION

[0021] The present invention addresses the need for optical transmissionsystems, apparatuses, and methods having increased flexibility andreliability. Optical systems of the present invention include amplifiersand methods configured to provide increased control over optical signalsbeing transmitted in the systems.

[0022] In various embodiments, the optical system and amplifiers areconfigured to achieve maximum signal channel power in a span downstreamof the transmitter and amplifier sites and to decrease the interactionbetween the wavelengths at high signal channel powers. In addition, thesystem can include various types of optical fiber positioned in thenetwork to provide for increased signal channel powers and higher gainefficiencies in the system.

[0023] In various embodiments, fiber Bragg gratings and other devicesare used to control the optical noise in the optical energy supply byoptical sources, such as semiconductor laser diodes, to provide forlower noise pump sources. The lower noise pump sources can be used tosupply optical energy for amplification to various optical amplifiers,such as Raman and erbium/rare earth doped fiber amplifiers and tocontrol the noise figure of the amplifier.

[0024] In addition, the system can include composite gain flatteningfilters that are used to modify the gain profile produced over aplurality of optical amplifiers. The composite gain flattening filterprovides flexibility in the operation of the individual amplifiers,which can be dynamically controlled to accommodate changes in the systemand to the optimize the performance of the composite filter.

[0025] Accordingly, the present invention addresses the aforementionedconcerns by providing optical systems apparatuses, and methods havingincreased performance and reliability. These advantages and others willbecome apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings for thepurpose of illustrating present embodiments only and not for purposes oflimiting the same, wherein like members bear like reference numeralsand:

[0027]FIGS. 1-2 show optical communication systems of the presentinvention;

[0028] FIGS. 3(a&b) show signal power versus distance curves in thepresent invention;

[0029]FIGS. 4-5 show optical amplifier embodiments of the presentinvention; and,

[0030]FIGS. 6-7 show Raman gain profiles.

DESCRIPTION OF THE INVENTION

[0031]FIG. 1 shows an embodiment of an optical system 10, in which anoptical energy pump source 12 is deployed between two optical nodes 14.The optical system 10 can be controlled by a network management system16 and pump sources 12 configured to operate in one or more seriallyconnected point to point links (FIG. 1) or in multi-dimensional networks(FIG. 2). The network management system 16 can communicate with thevarious nodes and elements in the optical systems 10 via wide area, datacommunication networks external to the system 10 and/or supervisoryoptical channels within the system 10.

[0032] The optical nodes 14 can be interconnected optically via guidedand unguided transmission media, which is typically optical fiberwaveguide 18. Optical amplifiers 20 typically will be provided alongoptical links 15 of sufficient length where amplification will berequired to overcome optical signal attenuation or proximate othercomponents to overcome component losses in the system 10.

[0033] One or more transmitters 22 can be included in the nodes 14 andconfigured to transmit information via the optical signals in one ormore information carrying signal wavelengths, or signal channels,λ_(si). Similarly, one or more optical receivers 24 can be provided invarious nodes 14, each configured to receive one or more signal channelsλ_(si) from the fiber 18.

[0034] The optical processing nodes 14 may also include other opticalcomponents, such as one or more add/drop devices 26 and optical andelectrical switches/routers/cross-connects 28 interconnecting thetransmitters 22 and receivers 24. For example, broadcast and/orwavelength reusable, add/drop devices, and optical andelectrical/digital cross connect switches and routers can be configuredvia the network management system 22 in various topologies, i.e., rings,mesh, etc. to provide a desired network connectivity.

[0035] The transmitters 22 used in the system 10 will generally includea narrow bandwidth laser optical source that provides an opticalcarrier. The transmitters 22 also can include other coherent narrow orbroad band sources, such as sliced spectrum sources, as well as suitableincoherent optical sources, such light emitting diodes and fiber lasers,as appropriate. Information can be imparted to the optical carriereither by directly modulating the optical source or by externallymodulating the optical carrier emitted by the source. Alternatively, theinformation can be imparted to an electrical carrier that can beupconverted using the optical carrier onto an optical wavelength toproduce the optical signal. Similarly, the optical receiver 24 used inthe present invention can include various detection techniques, suchcoherent detection, optical filtering and direct detection, andcombinations thereof. Employing tunable transmitters 22 and receivers 24in the optical nodes 14 in a network, such as in FIG. 2, can provideadditional versatility in the system 10.

[0036] The transmitters 22 and receivers 24 can be also connected tointerfacial devices 30, such as electrical and optical cross-connectswitches, IP routers, etc., to provide interface flexibility within, andat the periphery of, the optical system 10. The interfacial devices 30can be configured to receive, convert, and provide information in one ormore various protocols, encoding schemes, and bit rates to thetransmitters 22, and perform the converse function for the receivers 24.The interfacial devices 30 also can be used to provide protectionswitching in various nodes 14 depending upon the configuration.

[0037] Generally speaking, N transmitters 22 can be used to transmit Mdifferent signal wavelengths to J different receivers 24. In variousembodiments, one or more of the transmitters 22 and/or receivers 24 canbe wavelength tunable to provide wavelength allocation flexibility inthe optical system 10. In addition, the system 10 can also be configuredto carry uni- and bi-directional traffic.

[0038] Optical combiners 32 can be used to combine the multiple signalchannels λ_(si) into WDM optical signals, as well as multiple pumpwavelengths kpi for transmission in the fiber 18. Likewise, opticaldistributors 34 can be provided to distribute the optical signal to thereceivers 24 _(j) and optical signal and pump wavelengths λ_(pi) tomultiple paths. The optical combiners 32 and distributors 34 can includevarious multi-port devices, such as wavelength selective andnon-selective (“passive”), fiber and free space devices, as well aspolarization sensitive devices. The multi-port devices can variousdevices, such as circulators, passive, WDM, and polarizationcouplers/splitters, dichroic devices, prisms, diffraction gratings,arrayed waveguides, etc.

[0039] The multi-port devices can be used alone or in variouscombinations along with various tunable or fixed wavelength filters inthe optical combiners 32 and distributors 34. The filters can includevarious transmissive or reflective, narrow or broad band filters, suchas Bragg gratings, Mach-Zehnder, Fabry-Perot and dichroic filters, etc.Furthermore, the combiners 32 and distributors 34 can include one ormore stages incorporating various multi-port device and filtercombinations to multiplex, consolidate, demultiplex, multicast, and/orbroadcast signal channels λ_(si) and pump wavelengths λ_(pi) in theoptical systems 10.

[0040] In various embodiments, the optical systems 10 includetransmitters 22 that are configured to launch the optical signals belowthe nonlinear limits for the maximum signal launch power. Likewise,optical amplifiers 20 are configured such that the optical signals arenot amplified to the maximum signal power at discrete amplifier sites.Instead, the optical signals are amplified during transmission in thefiber 18 such that the maximum signal channel power in the link 15 isachieved downstream from nodes and amplifier sites using distributedamplification.

[0041] Amplification of the optical signals to maximum power duringtransmission generally is performed using stimulated Raman scattering ofoptical energy co-propagating with the signal channels. The opticalfiber 18 or other transmission media generally will be a silica-basedwaveguide or other material compositions that produces Raman scatteringof optical energy in the material resulting in Raman amplification ofthe signal channels λ_(si). However, various embodiments can includedoped transmission fiber or fiber designed for use with otheramplification techniques to provide for distributed amplification in thetransmission fiber.

[0042] By moving the location of the maximum channel power downstream ofthe transmitters 22 and the amplifiers 20, the physical distance betweenthe optical amplifiers 20 actually can be increased, while the effectiveamplifier spacing is decreased. FIGS. 3(a&b) depicts optical signalpower variations during transmission through the fiber 18 as a functionof distance. The curve in FIG. 3(a) depicts the variations in the signalchannel powers that are amplified by co-propagating Raman amplificationafter being launched into the fiber 18 and at discrete amplifier sites 1and 2. The distance designated as T in FIG. 3(a) shows the maximumamplifier spacing achievable by maximizing the signal launch power. Thiscan be contrasted to the amplifier spacing of the present invention asshown by the spacing between the launch point 0 and amplifier sites 1and 2.

[0043]FIG. 3(b) is similar to FIG. 3(a), except additional amplificationis provided via counter-pumped distributed amplification and/orconcentrated amplification at the amplifier site. As shown in FIG. 3(b),both co- and counter-pumping the fiber 18 can further increase thedistance between the launch point and the first amplifier and theamplifier before the next node 14.

[0044] Curve 1 in FIG. 3(b) depicts the signal power assumingdistributed Raman gain resulting from both co-pumping andcounter-pumping the transmission fiber 18 using an amplifierconfiguration similar to that shown in FIG. 4. The skilled artisan willappreciate that each pump source 12 shown in the drawings can beconfigured to include various numbers of pumps and combinations of pumpwavelengths.

[0045] Similarly, curve 2 in FIG. 3(b) shows the effect of including aconcentrated, or lumped, amplifier stage 36 at each amplifier site, suchas shown in FIG. 5, which includes two concentrated amplifiers 36 forexemplary purposes. The optical amplifier 20 can include one or moreserial and/or parallel amplifier stages, which may include combinationsof one or more, distributed and concentrated amplifier stages. Theoptical amplifiers 12 may also include remotely pumped doped fiber orRaman amplifying fibers 36 having different amplification andtransmission characteristics, e.g., dispersion, etc., than thetransmission fiber 14. The remotely pumped amplifying fiber 36 can bepumped with excess pump power supplied to provide Raman gain in thetransmission fiber 14 or via a separate fiber. In addition, the opticalamplifiers 20 can include lumped fiber amplifier stages operated in deepsaturation using pump power being supplied to other stages.

[0046] Other optical signal varying devices, such attenuators, filters,isolators, and equalizers can be deployed before, between, and aftervarious stages of the amplifier 12 to decrease the effective lossassociated with the devices. Similarly, signal processing devices, suchas add/drop devices, routers, etc. can be included proximate the variousamplifier stages.

[0047] Contrary to traditional optical systems, if the amplifier spacingis to be increased in various embodiments of the present invention, thesignal launch power will be decreased. Likewise, the signal poweremerging from the optical amplifier sites also will be decreased. Thesignal launch power is lowered to extend the distance from thetransmitter 22 and amplifier 20 at which the signal channels willachieve maximum power. The cumulative slope of the Raman gain and fiberattenuation versus distance will determine the location at which themaximum signal channel power will be achieved downstream of the nodes 14and amplifiers 20.

[0048] The nonlinear interaction limit on signal power and the minimumpower level required to provide a signal to noise ratio to allowdetection with an acceptable bit error rate define a transmission windowfor fiber. Employing various coding techniques, such as forward errorcorrection “FEC”, can expand the transmission window by enabling afinite number of transmission errors to be corrected followingtransmission. The number of transmission errors that can be correcteddepends upon the particular coding schemes used. Therefore, the systems10 can tolerate increased signal degradation resulting higher levels ofnonlinear interactions and lower signal to noise ratios.

[0049] Pump sources 12 generally include one or more optical sources, orpumps, configured to introduce optical energy, or pump power, in one ormore pump wavelength λ_(pi) bands into the fiber 18 to produce Ramangain in the optical signal wavelengths, or channels, λ_(si). The opticalsources are typically semiconductor laser diodes, the emission bandwidthand power of which is tailored to the specific system. Other narrow orbroad band, coherent and incoherent sources, such as optical noisesources, fiber lasers, light emitting diodes, etc. that can providesufficient optical energy also can be used in the system 10.

[0050] As shown in FIG. 6, the introduction of pump power in onewavelength/frequency band can produce Raman amplification of opticalsignal channels in another wavelength/frequency band. In fact, the pumpwavelengths λ_(pi) can be selected and the pump source 12 operated toallow for dynamic control over the amplification, or gain, profile ofthe signal channels being amplified. Exemplary optical amplifiers 20providing for dynamic control are described in U.S. patent applicationSer. Nos. 09/119,556 and 09/253,819, which are incorporated herein byreference.

[0051] As described in the incorporated references, the pump source 12within the amplifiers 20 can be configured to operate in concert withother optical amplifiers 20 in the system 10. For example, the opticalamplifiers can include pump sources 12 to provide for distributed and/orconcentrated amplification using counter-pumping and/or co-pumping.

[0052] In various embodiments, the fiber spans extending between thenodes 14 and amplifier 20 are co-pumped using a first set of one or morepump wavelengths λ_(p1i) and are counter-pumped using a second set ofone or more pump wavelengths λ_(p2i). The first and second sets of pumpwavelengths λ_(p1i) and λ_(p2i) are selected to minimize the gainvariation across the signal wavelength range as described in theincorporated references.

[0053] The gain profile of the distributed amplification can becontrolled in various embodiments by varying the pump power delivered ineach of the pump wavelengths counter-pumping of a span of fiber betweenamplifier locations. While the gain profile can be controlled, the noisefigure of the amplifier 20 will tend to be higher for signal channels atshorter wavelengths that are amplified correspondingly by the shorterpump wavelengths. As such, the signal to noise ratio of the signalchannels tends to degrade more rapidly at the shorter wavelength signalchannels, thereby limiting the overall transmission distance of thesignal channels.

[0054] In one aspect of the present invention, it was found thatselectively co-pumping the span in addition to counter-pumping the fiberspan can be used to vary the noise figure of the amplifier 20. Theco-pumping thereby provides a means to control the noise figure profileover the signal wavelength range, in addition to the gain profile. Inthese embodiments, co-pumping with optical energy in the shorter pumpwavelengths can be used to lower the noise figure for shorter wavelengthchannels relative to longer wavelength signal channels, while tending tolower the effective noise figure of amplifier for the entire signalchannel range.

[0055] In exemplary embodiments, the counter-pumped, second set of pumpwavelengths λ_(p2i) can be equally or unequally distributed across anentire pump wavelength range. Whereas, the first set of pump wavelengthsλ_(p1i) may not cover the entire pump wavelength range, but may belimited to one or more co-pump wavelengths in the shorter wavelength endof the range to control the noise figure. The co-pumped wavelengthsλ_(p1i) also can be used to provide some level of Raman amplification tothe shorter signal channel wavelengths and to the longer pumpwavelengths used to counter-pump the signal channels. For example, pumpwavelengths ranging from approximately 1410-1490 nm can be used tocounter-pump transmission fiber 18 to provide distributed Ramanamplification of signal channels λ_(si) in the 1520-1570 nm wavelengthrange. One or more pump wavelengths in the 1410-1430 range can be usedto co-pump the transmission fiber to control the amplifier noise figurefor the signal channels. Similarly, other signal channel ranges, such as1520-1630 nm, 1300-1310, etc. can be used to transmit signals dependingupon the attenuation of the particular fiber 18 in the system 10. Itwill be appreciated that more than one wavelength ranges can be used totransmit signal channels, each of which can be amplified using Ramanamplification supplied with pump power in a corresponding wavelengthrange.

[0056] The selection of pump wavelengths used in the system 10 willdepend upon the signal channel wavelengths and the type of fiber beingused in the system 10 and the various nonlinear interactions between thewavelengths. For example, single frequency pumps can used to controlnon-linear interactions between the pump and signal wavelengths and thelocation and intensity of mixing products in the system 10. In thepresent invention, low speed dithering below the signal bit rate, e.g.,<1 MHz, of the pump frequency can be used to minimize stimulatedBrillouin scattering (“SBS”) interactions and avoid non-linearinteractions, such as four wave mixing. High speed ditheringsignificantly above the channel bit rate, e.g., 5×, can also beemployed, if it is only necessary to minimize SBS as in the prior art.

[0057] While controlling the pump power supplied by the various pumpsources 12 provides increased flexibility in controlling the gainprofile of the amplifier 20, the discrete nature of the Raman gainassociated with each pump wavelength can introduce some variations intothe gain profile. The gain variations can accumulate, when amplifiers 20including discrete pump wavelengths λ_(pi) are cascaded in the system10.

[0058] In the present invention, a composite gain flattening filter 38in combination with a plurality of Raman amplifiers to collectivelyequalize gain variations from a desired gain profile, as shown in FIGS.7a-7 b. The composite Raman filter can be matched to and configured tocontrol the gain profile based on the composite pump wavelengths λ_(pi)of a plurality of Raman amplifiers. Unlike gain flattening filters ofthe prior art, the present invention does not attempt to perform gainfiltering at each amplifier. Instead, gain flattening filters 38 of thepresent invention are designed based on a composite gain variationprofile from a plurality of optical amplifiers 20, thereby reducing theextent and precision of the filtering required to control the gainprofile. In addition, the composite gain flatten filters 38 allow forthe gain in the individual amplifiers 20 to be varied without affectingthe performance of the filter 20, because the composite gain of theamplifiers 20 is being filtered. As such, the performance of the filters38 in present invention does not require each amplifier 20 to beoperated in a fixed mode to ensure proper control of the gain profile,as in the prior art.

[0059] In addition, the composite gain filtering does not require theprecision of individual amplifier filtering, because there are fewerdevices used in the system which reduces the accumulation of errors.This provides additional flexibility in the design of the filters 38. Infact, the gain profile of the amplifiers 20 being filtered can be variedto accommodate variations in the gain flattening profile of the filter38.

[0060] Periodic gain flattening also allows the amplifier 20 to beoperated dynamically to adjust for changes in the system performancewithout being limited to maintaining a specific gain shape at eachamplifier 20. For example, if a laser diode fails in one of theamplifiers, it may be necessary to vary the gain profiles in the otheramplifiers to compensate for the failed amplifier. Thus, each of theamplifiers in an amplifier chain being gain flattened by the filter 38can be operated to provide a different gain profile than during normaloperation, but the composite gain profile at the filter 38 could bemaintained. As such, it is possible to compensate for system changes andmaintain system performance through the use of a composite Raman gainflattening filter 38.

[0061] A limitation of co-pumped Raman amplification is that when thesignal channels are launched near maximum power, the pump wavelengthsand the signal wavelengths can cause interference with each other, whichis referred to as crosstalk interference. In saturated amplifiers, thesignal channels can cause a pattern dependent depletion of the opticalenergy in the pump wavelengths, which can then be transferred to othersignals. Crosstalk occurs in co-pumping scenarios to a greater extentbecause the walk-off between pump wavelengths and signal channels islimited to the dispersion rate in the fiber. Whereas, in counter-pumpedRaman amplifiers, pump depletion generally is averaged, because of therate at which the pump wavelengths counter-propagate relative to thesignal channels.

[0062] However, if the signal channel powers are reduced sufficiently,the co-pumped Raman amplifier can transition to unsaturated regime wherecrosstalk is substantially reduced. In this regime, co-pumped Ramanamplifiers can be used to provide additional gain in the fiber span. Thewalkoff between signal channels and the pump wavelengths also isaffected by the dispersion difference between standard and LD fibers.The dispersion profile in LD fibers are controlled by waveguide designand not material dispersion as in standard fiber. Thus, the dispersiondifference between pump and signals can be greater in some LD fibersthan in standard fiber.

[0063] In co-pumped Raman amplifiers, intensity noise in the pumpsources 12 can be transferred from the pump wavelengths onto the signalchannels more readily than when counter-pumped. Therefore, it generallyis desirable to use pump sources 12 that have low noise characteristics,such as single frequency diodes. Also, the pump sources 12 can generallybe constructed using fiber Bragg gratings in polarization maintaining(“PM”) fiber pigtails on diode lasers as discussed in the incorporatedapplications to control the emission wavelength range.

[0064] In one aspect of the present invention, the positioning of theBragg grating relative to the optical source, e.g. laser diode, can beused to control the noise spectra of the emitted optical energy. It isgenerally desirable to position the Bragg grating in the PM pigtail,such that relative intensity noise generated by relaxation oscillationsof the external cavity are at sufficiently high frequencies that thenoise will not be imprinted on the signal channels. For example,positioning the Bragg grating or other external cavity device close tothe output facet of the cavity can reduce the noise. In addition, theoutput facet of the laser cavity can be provided with an anti-reflective(“AR”) coating and/or angled to suppress relative intensity noise due tocompetition between external cavity modes and the internal lasing modesof the laser. Generally, it is desirable to minimize the relativeintensity noise associated with the pumps, for example, less than −130dB/Hz, to minimize the imprinting of noise onto the signal channels.

[0065] In addition, fiber Bragg gratings or other bandwidth controllingdevices can be used to control the emission wavelength range of theoptical sources, so the pump power can be efficiently combined throughthe pass band of WDM couplers and other WDM pass band devices. In thepresent invention, the pump source 12 in the amplifier 20 includes WDMcouplers having pass bands outside the pump wavelength range required toprovide Raman amplification to the signal channels being used in thesystem 10. For example, fused tapered WDM couplers can be cascaded tospan the desired pump wavelength range. Fiber Bragg gratings can be usedto control the emission spectra of the optical sources to be within thepass bands of the couplers. These embodiments can provide for ascalable, non-service interrupting expansion of the signal channelcapacity of the system 10. Pump wavelengths can be added to the pumpsource 12 in the unused pass bands to expand the amplification range ofthe amplifier 20 and the available signal channel wavelength rangeduring operation.

[0066] Additionally, the Raman gain achieved in the span is dependentupon the relative polarization of the signal channels and the pumpwavelengths. Polarization dependent gain can by reduced or eliminated bydepolarizing the pump light. Linearly polarized output from these pumpscan be coupled to polarization maintaining (“PM”) fiber with itselectric field vector polarized at 45 degrees to a polarization axis ofthe PM fiber to depolarize the output. Some higher power applicationsrequire multiple pumps to be multiplexed together, such as embodimentsshown in FIG. 5. This can be done using a polarization beamsplitter/combiner (“PBC”) 24 to combine the pump wavelengths. PM fibercan be placed after the polarization combiner 24 to depolarize the lightfrom both pumps. When the same pump wavelengths are combined using a PBCthe combined power is effectively depolarized. However, in the absenceof a depolarizer following the PBC, the failure of one of the twooptical sources will result in a linearly polarized output from the PBC.

[0067] The launched signal channel power and relative maximum signalchannel powers at the transmitters 22 and amplifiers 20 can be tailoredto accommodate variations in the nonlinear interaction limits across thewavelength range. For example, in LD fibers, the dispersion profile andnonlinear interaction profile strongly varies across the signal channelband. Therefore, the signal channels can be launched having varyingchannel powers, which can be compensated by varying the Ramanamplification of the channels accordingly. The net gain across onecomplete span between amplifiers can be made to be flat, but thenonlinear penalties can be minimized, by varying the relative signalchannel powers along the span.

[0068] As discussed in the incorporated applications, additionalamplification may be possible by co-pumping the fiber span with pumpwavelengths in the 1320-1410 nm range to amplify the counter-pumpwavelengths in the 1410-1490 nm range. As also discussed, a dopedsection of fiber, i.e., erbium, can be spliced into the span at anappropriate location, which will be pumped by remnant pump energy andprovide additional amplification. However, the inclusion of a dopedfiber could limit the signal channel wavelength range.

[0069] The maximum signal channel launch power in LD fibers issubstantially lower than in standard fiber. As such, it is necessary toachieve increased levels of Raman gain to achieve LD fiber systemperformance that is comparable to the performance of standard fibersystems. An advantage of LD fibers is that the Raman cross section isgenerally smaller than standard fiber; therefore, higher Raman gains areachievable for the same pump power in LD fibers. The improved Raman gainperformance tends to offset to some extent the lower launch powers.Also, because the LD fiber was designed to have very low dispersion, itcan reduce the number of dispersion compensating components and fibersrequired in the system.

[0070] It may be desirable to combine fiber types along a span betweennodes and/or amplifiers to obtain the benefits associated with eachfiber type. For example, standard fiber or higher dispersion fiber couldbe deployed as the transmission fiber 18 in regions of the system inwhich the signal channel power is high. Such regions include thetransmission fiber 18 following transmitters 22 and amplifiers 20 in thesystem 10. The use of high dispersion fiber in high power regions allowsfor higher maximum signal channel powers.

[0071] Analogously, LD fiber would be particularly useful in regionswhere the signal channel power is low. For example, LD fiber can beplaced before amplifiers and receivers in the system, where the signalpower is low. The inclusion of LD fiber regions would substantiallydecrease the overall dispersion in the system, which could substantiallyreduce or eliminate the need for DC components and fibers. Additionally,the higher Raman gain achievable in LD fiber would enable more efficientpump utilization and distributed amplifier performance.

[0072] In application, a first section extending from the transmitters22 and output of the amplifiers 20 could be a larger core fiber, such asSMF-28-like standard transmission fiber, that allows for higher signalpowers than in smaller LD fiber. Second section of smaller core fibers,such as the LD fiber, can be used in lower signal power regions, such asbefore receivers 24 and amplifiers 20, where the smaller core fiber canprovide increased Raman gain efficiencies. Other fibers can beinterleaved between the first and second fiber sections as may beappropriate to achieve various objectives in the systems.

[0073] The benefits associated with other fiber types could also beleveraged by appropriately positioning of fiber in the transmissionpath. Hybrid fiber types could be most cost effectively deployed at thetime a new fiber path is installed, particularly in festooned andundersea systems. In existing systems, retrofitting one or moredifferent fibers into an existing fiber plant may not be a costeffective proposition. However, in fiber paths that contain multiplefiber types, it may be possible to interleave the existing fibers, forexample at junction boxes, to produce a hybrid fiber having increasedperformance.

[0074] Those of ordinary skill in the art will appreciate that numerousmodifications and variations that can be made to specific aspects of thepresent invention without departing from the scope of the presentinvention. It is intended that the foregoing specification and thefollowing claims cover such modifications and variations.

1. (cancelled)
 2. The method of claim 4, wherein said co-propagatingincludes co-propagating optical energy in the second pump wavelengthrange that overlaps with shorter wavelengths in the first wavelengthrange.
 3. The method of claim 2, wherein: said transmitting includestransmitting optical signals in a signal wavelength range from 1530-1570mm; said counter-propagating includes counter-propagating optical energyin the first pump wavelength range from 1410 to 1480 nm; and, saidco-propagating includes co-propagating optical energy in the second pumpwavelength range from 1410 to 1430 nm.
 4. A method of amplifying opticalsignals comprising: transmitting optical signals in a transmission mediaconfigured to transmit and provide Raman amplification of the opticalsignals; counter-propagating optical energy in the transmission media ina first pump wavelength range to produce Raman amplification of theoptical signals, wherein the Raman amplification has a correspondingnoise figure profile over an optical signal wavelength range; and,co-propagating optical energy with the optical signals in a second pumpwavelength range to vary the noise figure profile of the Ramanamplification produced by said counter-propagating optical energy overat least a portion of the optical signal wavelength range; impartinginformation onto a plurality of signal channels; determining a nonlinearinteraction limit for the signal channels in the transmission media;controlling the signal channel launch power to produce a level ofnonlinear interaction below the nonlinear interaction limit when thesignal channels are combined into the optical signals as a WDM opticalsignal; combining the signal channels into a WDM optical signal; saidtransmitting including launching the WDM optical signal into thetransmission media; and, said co-propagating including co-propagatingoptical energy with the signal channels in the transmission media toamplify the signal channels and achieve a maximum signal channel powerbelow the nonlinear interaction limit at a point in the transmissionmedia downstream from where the WDM optical signal was launched.
 5. Themethod of claim 4, wherein: said controlling includes, controlling thesignal channel launch power to vary inversely with the distance to overwhich the signal channels are to be transmitted; and, saidco-propagating includes co-propagating optical energy at a power thatvaries directly with the distance over which the signal channels are tobe transmitted.
 6. The method of claim 4, wherein said method includescounter-propagating optical energy to provide Raman gain in the signalchannels in the transmission media.
 7. The method of claim 4, wherein:said imparting includes imparting the information onto signal channelsin the 1520-1630 nm wavelength range; and, said co-propagating includesco-propagating optical energy in the 1400-1520 nm wavelength range toprovide Raman gain to the signal channels.
 8. The method of claim 4,further comprising: providing at least one pump source configured tosupply the optical energy in one or more pump wavelengths having a lowrelative intensity noise.
 9. (cancelled)
 10. The method of claim 8,wherein said providing includes providing the at least one pump sourceincluding a laser having a Bragg grating positioned to relative to thelaser to form an external cavity to control relative intensity noise ofthe optical energy.
 11. The method of claim 10, wherein said providingincludes providing the laser with an output facet that includes at leastone of an anti-reflective coating and an angled output facet. 12.(cancelled)
 13. The system of claim 16, further comprising: a pluralityof optical amplifiers disposed between said optical nodes and configuredto optically amplify the optical signals passing between said nodes,wherein said optical amplifiers include pump sources configured toprovide optical energy in a pump wavelength range to produce Ramanamplification of the optical signals; and, a gain flattening filterconfigured to filter a composite gain profile produced by said pluralityof optical amplifiers and impart a desired gain profile to the opticalsignals.
 14. The system of claim 13, wherein: said plurality of opticalamplifiers includes a plurality of different pump sources providingdifferent amounts of optical energy in pump wavelengths within the pumpwavelength range; and, said Raman gain flattening filter is configuredto filter the composite gain profile produced by the pump wavelengths.15. The system of claim 13, wherein said plurality of optical amplifiersincludes at least one of distributed and concentrated Raman amplifiers.16. An optical system comprising: at least two optical nodes configuredto transmit and receive optical signals between said nodes via atransmission media configured to provide Raman amplification of theoptical signals; a counter-pump source configured to counter-propagateoptical energy in the transmission media in a first jump wavelengthrange to produce Raman amplification of the optical signals, wherein theRaman amplification has a corresponding noise figure profile over anoptical signal wavelength range; a co-jump source configured toco-propagate optical energy with the optical signals in a second pumpwavelength range to vary the noise figure profile of the Ramanamplification produced by said counter-propagating optical energy overat least a portion of the optical signal wavelength range; wherein atleast one of said processing nodes is configured to launch opticalsignals into said transmission media at a power below a nonlinearinteraction limit of said transmission media; at least one pump sourcepositioned to co-propagate optical energy with the optical signals insaid transmission media to amplify the signal channels and achieve amaximum signal channel power below the nonlinear interaction limit at apoint in the transmission media downstream from said first processingnode.
 17. The system of claim 16, wherein: said system includes at leastone optical amplifier configured to amplify the signal channels passingthrough said amplifier; said at least one pump source includes at leastone pump included with said optical amplifier and configured toco-propagate optical energy with the optical signals to amplify thesignal channels and achieve a maximum signal channel power below thenonlinear interaction limit at a point in said transmission mediadownstream from said optical amplifier.
 18. (cancelled)
 19. The systemof claim 16, wherein said system includes launching signal channels inthe 1520-1630 nm transmission range; and, said at least one pump sourceprovides optical energy in the 1400-1520 nm wavelength range to amplifythe signal channels via stimulated Raman scattering.
 20. (cancelled) 21.The system of claim 16, wherein said transmission media includes atleast first and second fiber sections, wherein said first fiber sectionhas a first core diameter and is disposed between the node transmittingoptical signals and the second fiber section, and, said second fibersection has a second core diameter that is less than the first corediameter and is disposed between the node receiving optical signals andthe first fiber section.
 22. The system of claim 21, further comprisingat least one optical amplifier disposed between said at least twooptical nodes and configured to amplify optical signals passing from oneof the second fiber sections to one of the first fiber sections.
 23. Anoptical system comprising: at least two optical nodes configured totransmit and receive optical signals between said nodes via atransmission media configured to support Raman amplification of theoptical signals, wherein said transmission media includes transmissionfiber having a plurality of first and second fiber sections, wherein thefirst fiber section has a first core diameter, and the second fibersection has a second core diameter that is less than the first corediameter; a plurality of optical amplifiers disposed between said atleast two optical nodes, wherein at least one of the plurality ofoptical amplifier between one of the first and second fiber sections andconfigured to propagate optical energy in a first pump wavelength rangein at least one of the first and second fiber sections to produce Ramanamplification of the optical signals passing through the at least one ofthe fiber sections.
 24. The system of claim 23, wherein: one of thesecond fiber sections is connected to one of the optical nodes that isconfigured to receive optical signals from the one of the second fibersection; and, one of the first fiber sections is connected to one of theoptical nodes that is configured to transmit optical signals into theone of the first fiber sections.
 25. The system of claim 23, wherein:each of the plurality of optical amplifiers has an input connected toone of the second fiber sections and an output connected to one of thefirst fiber sections and is configured to counter-propagate opticalenergy in the transmission media of the second fiber section in a firstpump wavelength range to produce Raman amplification of the opticalsignals passing through the second fiber section.